Tate Modern architect Herzog and de Meuron has reduced the scope of its London office after the departure of office director Michael Casey.

Casey left at the end of March to set up his own practice and will not be replaced by another associate. The highest ranking member of the reduced office will be a project architect.

A spokesperson for the Swiss practice, which will unveil its designs for the £140m extension to the Tate Modern Gallery in London next week, said the firm would not have an associate in London but would move different senior figures back and forth.

She said: “There is no associate assigned to London. The number of people there will depend on the number of projects and what their status is.”

She added that Jacques Herzog and Pierre de Meuron would personally take control of the Tate extension scheme.